The present invention refers to an improved performance welding bar.
In machines used for the welding of thermoplastic material, such as those used for the production of bags welded at one end, so-called xe2x80x9copen-mouthxe2x80x9d, or at both ends, made from a tubular plastic material or similar, transversal and/or longitudinal pairs of heated welding bars are used, simply called xe2x80x9cweldersxe2x80x9d.
The structures of these welders are presently made from cast iron as there is a need to have uniform heat along the entire bar and to have a specific weight which acts on the thermoplastic film of the entire area to be welded. Furthermore, the cast iron must be treated and smoothened in order to guarantee its superficial linearity.
However, it is to be taken into consideration that this kind of welder acts on the film according to a vertical backward and forward movement, biting it for a linear passsage, until it welds and cuts thanks to the presence of an interposed blade or even just thanks to the hot and sharpened welder in its striking area. The intervention of the two upper and lower bars is defined in practice as a xe2x80x9cblowxe2x80x9d or a xe2x80x9cstrikexe2x80x9d and the present machines work well even up to around 200-250 blows or strikes per minute.
This means that the machine and the welding group work outstandingly when the working speed, or the advancement speed of the film to be welded and the so-called xe2x80x9cblowsxe2x80x9d, for example, for the production of bags, are within the limits indicated above.
In modern practice, the trend is to accelerate production more and more by increasing the number of blows and therefore the number of bags produced. A large quantity of blows leads to two types of problems: maintaining the temperature in the welding bars constant throughout the entire welding cycle for hours and hours and the remarkable mass of the welding bars to be moved quicker than before.
As is known, such bars are raised and lowered backwards and forwards between themselves thanks to the presence of complex control mechanisms, normally placed in the lower part of the machine of welding of thermoplastic materials.
As these mechanisms are also made up of toothed components they cannot undergo outstanding inertia (accelerations, shutdowns, turnarounds, accelerations etc) and be turned at high speeds otherwise they would wear out very quickly and would result in the breakage and shutdown of the machine.
Furthermore, in the present machines around the cast iron bar there is a teflon band or a similar antigluing material to avoid the thermoplastic material film from sticking to the cast iron welder, considering the temperature. This band must be rotated after a certain number of blows or welds as it is also subject to wear.
Furthermore, the presence of this band leads to appropriate supports of the same as well as a hooking device on each bar.
Box-shaped welding bars with a rectangular section made from a light alloy with chromium nickel straps on the end overlooking the other bar, are also already known. Each strap is heated up like a resistance and it determines the welding and the simultaneous cutting of the thermoplastic material being treated. Even in this case the presence of a teflon band must be provided for, or a similar antigluing material that twists around the bar and especially the chromium nickel strap to avoid that the thermoplastic film sticks to the heated chromium nickel strap itself that welds, considering the temperature.
Therefore an objective of the present invention is that of identifying a solution that allows for the most simple and most functional solution possible, even resolving the complex technical problem previously indicated and highlighted.
Another objective is that of creating a bar which solves all problems related to the high mass of the present iron cast bars, minimizing the mass and the members in movement, fully satisfying the function of having a correct temperature and a correct planarity of the work surfaces.
Another objective is that of avoiding any production faults whatsoever even at the high speeds at which the machine operates.
According to the present invention these and other objectives are achieved by providing an improved performance welding bar which is placed within a welding machine for the welding of thermoplastic material, suitable for each of the two lower (11) and upper (12) welding bars, among said bars (11, 12) said material is moved along by at least one pair of lower (14) and upper (15) supply rollers, placed on top of each other, said two welding bars (11, 12) being moved backward and forwards between themselves by a control group (20), wherein said welding bar (11, 12) is made from a magnesium-zinc aluminum alloy, the weight percent of zinc is 5.0-6.5% and the weight percent of magnesium is 2.0-3.0%.